This invention relates to a hydrogen storage composition, methods for its manufacture and a method of providing a source of hydrogen.
Although metal hydrides offer a safe and convenient method for hydrogen storage, their practical application as hydrogen carriers still remains limited. This is caused by the fact that most applications require that metal hydrides exhibit in addition the following properties: high hydrogen capacity, fast kinetics of hydriding/dehydriding and suitable ranges of operational pressures and temperatures. Moreover, the hydrides should consist of inexpensive and available elements in order not to excessively raise the cost of hydrogen storage. Although the existing variety of metal hydrides offers a large spectrum of various properties which are suitable for hydrogen storage, so far none of the hydrides fulfills all the requirements. For example, FeTi or LaNi5 can operate at room temperature at reasonable hydrogen pressures, but their low hydrogen capacity in practice excludes the use of these hydrides in transportation, or in portable hydrogen storage containers. Vxe2x80x94Ti-based materials exhibit higher hydrogen capacities (up to 2 wt. %), but their cost is too high for any large-scale application.
Magnesium and magnesium-based alloys are the prime candidates for hydrogen storage amongst the existing metal hydrides. Magnesium forms a hydride (MgH2) which provides very high hydrogen capacity which at 7.6 wt. % is the highest of all metal hydrides with reversible performance. Additionally, the enthalpy of the hydride formation is large at 75 kJ/mole, which makes magnesium attractive for thermal energy storage. These features, combined with the very low cost and abundant accessibility of magnesium, suggest an excellent potential for hydrogen-related applications. However, to date magnesium hydride has been of no use for practical hydrogen storage because the reaction of hydriding/dehydriding is very slow and can be performed only at very high temperatures. In practice MgH2 cannot be formed at ambient conditions, and high temperature and high hydrogen pressure are required for the reaction to occur. Moreover, the reaction of hydrogenation is usually blocked by surface oxidation of the material. In order to overcome this problem magnesium has to be activated prior to hydrogenation. Effective activation of magnesium as, for example, in [1] consists of several cycles of annealing at 400xc2x0 C. in vacuum and in hydrogen, followed by annealing for several hours at 400xc2x0 C. in vacuum. However, even after such activation, hydrogenation at a temperature of 350xc2x0 C. and at a hydrogen pressure of 30 bars was not sufficient to initiate formation of magnesium hydride within 48 hours of annealing [1]. In practice, in order to achieve absorption rates for magnesium of the order of hours, the hydrogenation temperature should be higher than 400xc2x0 C. at a hydrogen pressure of at least 30 bars [2, 3]. In addition, magnesium has a low equilibrium pressure of desorption and therefore desorption of hydrogen from magnesium hydride has to be performed also at very high temperatures, usually 350xc2x0-400xc2x0 C.
An additional important feature of metal hydrides is that the reaction of dehydrogenation has an endothermic character. On one hand, this is very advantageous because it provides safety in the use of metal hydrides. In order to initiate desorption, the heat of the reaction has to be delivered to the system. Therefore desorption of hydrogen from the hydride can be performed with full control, avoiding excessive, uncontrollable production of free hydrogen. On the other hand, however, this feature poses important technical problems. In practice, the rate of hydrogen desorption can be significantly reduced by the very poor thermal conductivity of the hydride. In order to overcome this problem several technical solutions have been proposed, which in general involve improvement of heat transfer by designing special reaction beds [4, 5, 6].
Efficient metal hydride beds have various types of heat-transferring media incorporated into the bed. In practice, such media could be, for example, a heat sink in the form of rods, plates, wires or foams, made of a material with excellent thermal conductivity, for example, copper, aluminum or graphite. Such a heat sink, or a binder, or conductive matrix, can significantly increase the rate of the hydriding/dehydriding reaction. However, the resulting enhancement in the reaction kinetics is achieved always at the cost of the effective hydrogen capacity. The total weight of the reaction bed, including binders or heat sinks, increases substantially and the effective hydrogen capacity is in practice significantly reduced.
In response to the above drawbacks of metal hydrides, the present invention addresses simultaneously both problems: performance of metal hydride and heat-transfer limitations. As a result a viable metal hydride system is obtained, with high hydrogen capacity, fast performance and low cost.
It is an object of the invention to provide a hydrogen storage composition.
It is another object of the invention to provide a method of producing a source of hydrogen gas.
It is a further object of the invention to provide methods of producing the composition of the invention in the hydrogenated state.
In one aspect of the invention there is provided a hydrogen storage composition having a hydrogenated state and a dehydrogenated state, wherein (a) in said hydrogenated state said composition comprises (i) a metallic hydride having a metallic component which reversibly forms said hydride, and (ii) a metallic heat transfer medium in intimate contact with said hydride and effective to transfer heat to said hydride for dehydrogenation of said hydride, and (b) in said dehydrogenated state said composition comprises (iii) at least one intermetallic compound of said metallic component of said hydride and said metallic medium.
In particular, in the hydrogenated state, the hydride and the metallic medium are in fine particle form, preferably composed of particles having a size below 10 microns, more preferably below 100 nanometers, still more preferably below 50 nanometers and especially below 10 nanometers.
In another aspect of the invention there is provided a method of providing a source of hydrogen gas comprising liberating hydrogen from a composition of the invention as described above, at an elevated temperature, with transfer of heat to said hydride by said metallic heat transfer medium, removing said liberated hydrogen and regenerating said hydrogenated state by exposing said dehydrogenated state to hydrogen gas.
In other aspects there are provided methods for producing a composition of the invention.
In one embodiment of the latter aspect of the invention the method comprises mechanically alloying, rapidly solidifying or casting an intimate mixture of a metallic component which reversibly forms a metallic hydride and a metallic heat transfer medium, and hydriding said mixture with formation of the hydride of said metallic component while maintaining said metallic medium in non-hydrided form.
In another embodiment the method comprises mechanically alloying, rapidly solidifying or casting an intimate mixture of a metallic hydride having a metallic component which reversibly forms said hydride and a metallic heat transfer medium.
In still another embodiment the method comprises mechanically alloying, rapidly solidifying or casting an intermetallic compound of a metallic component which reversibly forms the metallic hydride and a metallic heat transfer medium and hydriding said intermetallic compound to form a hydride of said metallic component while maintaining said metallic medium in non-hydrided form.
The essence of the invention is that a heat-transfer medium which forms a heat-conducting binder or matrix, is made an integral part of the metal hydride system and plays a crucial role as a reagent in the hydrogenation/dehydrogenation reaction.
In the hydrogenated state of the system, the binder or matrix does not form a hydride but remains in a metallic state, thus preserving its excellent thermal conductivity. In the desorbed state, however, the binder or matrix undergoes a reaction with the basic metal component of the hydride. As a result of this reaction, both thermodynamic and kinetic properties of the main metal hydride are changed and a much better hydriding/dehydriding performance is obtained. The sequence of changing role by the binder or matrix is repeated in subsequent reversible cycles of hydrogenation and dehydrogenation of the system.
The metal hydride component in the system is suitably magnesium hydride. Magnesium hydride provides very high hydrogen capacity, having the highest reversible capacity of all metal hydrides, moreover, it is inexpensive and abundant, although normally magnesium hydride exhibits kinetic and thermodynamic limitations, as described above.
Aluminum is the preferred heat-transfer medium. Aluminum has an excellent thermal conductivity and is recognized as one of the best heat-transfer media, along with copper. As a binder or matrix aluminum does not play an active role in hydrogenation of metal hydrides, since it does not form reversible aluminum hydrides.
Generally the proportions of the metal of the metal hydride, and the heat transfer medium are such that the intermetallic compound or compounds formed in the dehydrogenated state exploit the total content of the two metals, with there being no free metal. In general there is employed 40 to 70%, preferably 45 to 65% of the metal hydride, for example, magnesium and 30 to 60%, preferably 35 to 55% of the heat-transfer medium, for example, aluminum to a total of 100%.
In the hydrogenated state, the system consists of two components, for example, MgH2 and Al, which are in physical contact, for the preferred case in which the hydride is magnesium hydride and the heat transfer medium is aluminum. During endothermic desorption of magnesium hydride, aluminum acts simply as a heat-transfer medium. For the desorption, i.e. for the dissociation of MgH2, a significant amount of heat has to be provided. The presence of aluminum plays a substantial role in speeding up this process, because MgH2 is a very poor heat conductor and alone cannot be sufficiently effective in heat transfer.
After the desorption is completed, the problem of heat-transfer is much less significant. In fact, it is even the opposite: slower heat removal from the absorbing material actually enhances the absorption reaction because it occurs at effectively higher temperature. Moreover, the metal of the metal hydride is at this stage in the metallic form and has much better thermal conductivity. Therefore, the role of the binder or matrix is less important for hydrogen absorption.
After the hydrogen desorption stage of the hydrogenation/dehydrogenation cycle i.e. after hydrogen desorption from MgH2, the aluminum changes its role. After the desorption, aluminum is not just a binder or matrix any more, but becomes an integral part of the system and a crucial reagent. The key point is that after the dissociation of MgH2, magnesium does not regain its elemental form, according to the dissociation reaction:
MeH2Mg+H2
but instantly reacts with aluminum to form distinct magnesium-aluminum phases.
Formation of Mgxe2x80x94Al phases is the basic difference between the present invention and the common situation when the binder or matrix and metal hydride remain separate and do not chemically interfere over the whole hydrogenation/dehydrogenation cycle. Moreover, the reaction in the present case changes the thermodynamic and kinetic properties of the system and the performance of the hydride is substantially improved.
Thus, the present invention proposes a new approach to the problem of heat transfer in the reaction bed of metal hydrides. Instead of a foam, binder or matrix acting solely as a heat sink in the reaction bed, a heat-transfer medium is introduced as an integral and crucial part of the metal hydride system. The heat transfer medium acts as a common heat sink, but only at a certain stage of the hydrogenation/dehydrogenation cycle, i.e., after absorption of hydrogen. In addition, the heat transfer medium acts as an important reagent in the desorbed state and changes thermodynamic properties of the system. As a result, the system exhibits very fast kinetics of absorption and desorption at medium temperatures, in contrast to the conventional magnesium hydride. In this way most problems of the metal hydride performance are practically solved: the problem of heat transfer in the hydrogenated state, slow kinetics and high temperature operation.
The system takes advantage of high capacity as a result of high content of MgH2, and at the same time the presence of Al gives improved heat-transfer and modified hydrogenation properties. When taking into account the hydrogen capacity of the metal hydride system including the heat transfer medium, the system of the invention provides the record weight capacity of all practical hydrides, with a total, reversible hydrogen content of 3.5-4.5 wt. %.
a) Chemical Composition of the System
The reaction between magnesium and aluminum requires first of all that the amounts of both elements in the system allow for the formation of Mgxe2x80x94Al phases. According to the phase equilibrium diagram (Binary Alloy Phase Diagrams, ed. American Society for Metals, Metal Park, Ohio, 1986, Vol. I, p. 129) there are several phases in the Mgxe2x80x94Al system: xcex2-(Al3Mg2), xcex3-(Mg12Al17), R-phase. The R-phase (often designated xcex5) is of composition 42 at % Mg. In addition, several metastable phases were also reported in this system, for example, Al2Mg, xcex31-(Al12Mg17), MgAl. The results show that by changing both the composition and the microstructure of the material, it is possible to form each of the above phases as an intermediate stage of the hydrogenation in an Mgxe2x80x94Al system. In some cases new, unknown phases were formed.
In addition, the reaction occurs not only for a strictly stoichiometric composition. Although the phases are in most cases xe2x80x9cline compoundsxe2x80x9d, the applied techniques of material fabrication, described below, may change solid-solubility regions of the phases. Therefore the phases can be formed in much wider composition ranges than these predicted from the phase-equilibrium diagram.
The reaction product does not have to be a single-phase material. For certain compositions in the Mgxe2x80x94Al system, the reaction product can consist of two or more phases with either stable or metastable character. Such a multi-phase material exhibits an accordingly modified hydrogenation behaviour, for example, a multi-stage character of the plateau of equilibrium pressure. However, even in the multiphase material the role of aluminum as a heat-transfer medium remains unaffected, since the whole amount of aluminum is always used as a binder in a hydrogenated state, independently of the phase composition.
If the content of Mg is higher than that necessary for the formation of the respective Mgxe2x80x94Al phases, an excess amount of unreacted Mg is present after desorption. In such a case Mg can still be active in the hydrogenation/dehydrogenation process, but its hydriding properties are not affected by the reaction with Al.
b) Microstructure
Microstructure plays a significant role in the present invention. A key condition for the effective performance is that both magnesium, or magnesium hydride and aluminum are in close proximity, allowing for a fast solid-state reaction upon dehydrogenation. On the other hand, aluminum has to form a network of conductivity paths for effective heat transfer in the absorbed state. Therefore, the optimum microstructure of the system consists of the two phases being in physical contact, with the interface area expanded as much as possible, in order to enhance the reaction rate. In practice, this means that the microstructure should consist of very fine particles or layers of the two phases, Mg or MgH2 and Al. Although relatively good results can be obtained when the two phases are of the size of micrometers, the best performance is obtained when the phases are on the scale of nanometers or tenths of nanometers. In practice, two kinds of microstructures can be equally effective: one consisting of small particles of both phases being in physical contact and the other consisting of particles containing fine precipitates of both phases.
c) Performance
In addition to the great enhancement of heat transfer within the material, the action of aluminum as an integral part of the system changes the whole hydrogenation performance. For some reason, when magnesium is not allowed to form a separate elemental phase after desorption, but reacts instantly with aluminum, the whole process of hydrogenation/dehydrogenation of MgH2 is much faster and can occur at much lower temperatures. Although this phenomenon is not being fully understood yet, some important aspects can be considered.
Hydriding of magnesium with and without the presence of active aluminum is different from both the thermodynamic and the kinetic point of view, and in consequence changes the practical behaviour of the system.
First of all, formation of magnesium hydride exhibits different thermodynamics during the dispropoitionation reaction, i.e., when magnesium comes from the compound, for example, MgAl or xcex3-(Mg12Al17), than in the case of hydrogenation of elemental magnesium. Although this phenomenon is not really understood, it is clear that it involves change in the bonding energy of the hydride and in the equilibrium properties of the system. In the case of Mgxe2x80x94Al phases, equilibrium pressure changes significantly. The plateau pressure is shifted substantially towards higher pressures, as compared to pure magnesium hydride. The extent of the change depends on which Mgxe2x80x94Al phase is formed. Results show that the highest value of the plateau pressure occurs in the case of the metastable MaAl phase, for which it was about three times higher than that of pure MgH2 at 280xc2x0 C. As a result, the system can operate, especially for desorption, at much lower temperatures then the conventional magnesium hydride. Temperature ranges normally inaccessible for magnesium, i.e., below 300xc2x0, become viable for the Mgxe2x80x94Al system and the system can effectively operate for both hydrogenation and dehydrogenation at 200xc2x0-280xc2x0 C. Moreover, the reaction is fast, not being retarded by the problems of oxidation and inactive surfaces of the magnesium or magnesium hydride. Formation of MgH2 is somehow much easier when magnesium is taken, possibly in a specific, very active form, from the Mgxe2x80x94Al compound, than from elemental magnesium particles. Therefore, in the present case the rates of both absorption and desorption are significantly higher and activation of the material is not necessary at all, in strong contrast to the conventional magnesium hydride.
d) Methods of Fabrication
The above microstructure can be obtained in many different ways. First of all, a variety of starting materials can be used. In general, the following starting components can give the requires microstructure
i) a mixture of magnesium and aluminum;
ii) a mixture of MgH2 and Al; and
iii) an already formed Mgxe2x80x94Al phase, stable or metastable.
In each case the best results are obtained when the starting components are in the nanocrystalline form, or the nanostructure is obtained later, in the course of the process, although it is not necessary for the reaction to occur.
There is a variety of methods to be used in order to obtain the right microstructure. Amongst them mechanical alloying or rapid solidification are most suitable, although simple casting could also be effective in production of Mgxe2x80x94Al phases. Ball-milling or grinding of the powders of Mg or MgH2 and Al is very useful in producing the required fine powders of the starting material.